The present invention relates to a discharge lamp comprising a translucent ceramic tube dosed with a metal halide and more particularly to a discharge lamp having a raised output.
An arc tube of this kind of a discharge lamp comprises a tube body which is made of translucent ceramic such as polycrystalline alumina and is tapered at both ends thereof to form a narrow tube portion at an end portion there of, and an electrode lead which is connected to an electrode and inserted in and sealed to the narrow tube portion with a sealing glass.
However, it was extremely difficult for this kind of a discharge lamp to provide the arc tube with an output as high as not lower than 150 W. The reason is as follows. In order to provide a higher output, the diameter of the tube body must be increased to prevent the temperature of the tube body from rising to abnormally high temperature. This makes the difference in diameter between the narrow tube portion of the tube body and the other portions considerably great, producing a sharply bent portion. Firstly, ceramic can hardly be formed into this shape, adding to cost. Further, even if this difficulty can be overcome, the temperature of the bent portion reaches an extremely high value during lighting of discharge lamp, making the bent portion more subject to cracking due to thermal impact. On the contrary, when the diameter of the narrow tube portion is increased, the gap between the narrow tube portion and the electrode lead increases, adding to the thickness of the sealing glass layer with which they are sealed to each other and hence raising a problem that the sealing glass layer can crack.
Therefore, an object of the invention is to improve the structure of the tube body of the arc tube, making it possible to provide a discharge lamp with a higher output, and prevent the occurrence of crack due to heat cycle, making it possible to prolong the life of discharge lamp.
In order to solve the foregoing problems, the inventors made extensive studies of shape of main tube body of arc tube. As a result, it was found that the output as high as not lower than 150 W and the prolongation of life can be together realized by arranging the main tube body in a form having a large diameter portion, a tapered portion disposed on both sides of the large diameter portion having a smaller diameter toward the forward end thereof and a small diameter portion connected to the forward end of the tapered portions wherein the tapered portion and the small diameter portion are connected to each other with a border having a curvature radius of not smaller than 2 mm.
The greater the curvature radius of the border of the tapered portion with the small diameter portion is, the more can be relaxed the thermal stress concentrated thereonto and can be inhibited the occurrence of crack even if the lamp output is high. From this standpoint of view, the curvature radius of the border of the tapered portion with the small diameter portion is more preferably not smaller than 5 mm. The curvature radius of the border is preferably greater but is preferably not greater than 12 mm, particularly not greater than 9 mm.
Further, the arrangement is more desirable such that a terminal plate of ceramic is fitted in and airtightly fixed to the interior of the small diameter portion of the main tube body, the terminal plate has an arrow tube of ceramic piercing there through and airtightly fixed thereto and the narrow tube has an electrically-introducing member with the electrode inserted and airtightly sealed thereto with a sealing glass. In this arrangement, the diameter of the small diameter portion can be increased, making it possible to reduce the angle of the tapered portion accordingly. This means that the wall surface of the tapered portion can be kept away from the electrode, making it possible to prevent the tapered portion and hence the border of the tapered portion with the small diameter portion from rising in its temperature and thus making it possible to provide a discharge lamp with a higher output. Further, the rise of temperature of the foregoing border can be inhibited, making it possible to effectively prevent the occurrence of crack and enhance the reliability of the sealed portion of the electrically-introducing member in the narrow tube. Thus, the life of the discharge lamp can be further prolonged. However, the present invention is not limited to the foregoing structure comprising a terminal plate fitted in a small diameter portion. Another structure may be employed such that the electrically-introducing member is directly inserted in the small diameter portion.
It is more desirable that the thickness of the foregoing terminal plate be from not smaller than 2 mm to not greater than 3 mm. This is because when the thickness of the terminal plate falls below 2 mm, the air tightness of the terminal plate with the narrow tube can be difficultly kept fair. On the contrary, when the thickness of the terminal plate exceeds 3 mm, the heat capacity of the terminal plate increases, producing a great temperature difference in the ceramic tube and hence causing crack on the ceramic tube. It is further desirable that the protruding length of the electrode represented by the distance between the end of the small diameter portion and the forward end of the electrode in the arc tube be from not smaller than 3 mm to not greater than 6 mm. This is because when the protruding length of the electrode falls below 3 mm, the temperature of the portion sealed with a sealing glass rises too much, causing the sealed portion to crack due to sudden thermal expansion developed by the repetition of switching of lamp on and off. On the contrary, when the protruding length of the electrode exceeds 6 mm, the temperature of the interior of the narrow tube can difficultly be raised, making it difficult to provide sufficient luminous characteristics.